This invention relates to a method for forming a thin polymer layer on a substrate. More particularly, this invention relates to an apparatus and method for forming p-xylylene from liquid compounds during fabrication of integrated circuits.
In the construction of integrated circuits, device geometries are constantly shrinking, resulting in an increase in parasitic capacitance between devices. Parasitic capacitance between metal interconnects on the same or adjacent layers in the circuit can result in crosstalk between the metal lines or interconnects and in a reduction of the response time of the device. Lowering the parasitic capacitance between metal interconnects separated by dielectric material can be accomplished by either increasing the thickness of the dielectric material or by lowering the dielectric constant of the dielectric material. Increasing the thickness of the dielectric materials, however, does not address parasitic capacitance within the same metallized layer or plane.
As a result, to reduce the parasitic capacitance between metal interconnects on the same or adjacent layers, one must change the material used between the metal lines or interconnects to a material having a lower dielectric constant than that of the materials currently used, i.e., silicon dioxide (SiO2), k≈4.0.
Jeng et al. in xe2x80x9cA Planarized Multilevel Interconnect Scheme with Embedded Low-Dielectric-Constant Polymers for Sub-Quarter-Micron Applicationsxe2x80x9d, published in the Journal of Vacuum and Technology in June 1995, describes the use of a low dielectric constant polymeric material, such as parylene, as a substitute for silicon dioxide (SiO2) between tightly spaced conductive lines or other strategically important areas of an integrated circuit structure. Parylene, a generic name for thermoplastic polymers and copolymers based on p-xylylene and substituted p-xylylene monomers, has been shown to possess suitable physical, chemical, electrical, and thermal properties for use in integrated circuits. Deposition of such polymers by vaporization and decomposition of a stable cyclic dimer, followed by deposition and polymerization of the resulting reactive monomer, is discussed by Ashok K. Sharma in xe2x80x9cParylene-C at Subambient Temperaturesxe2x80x9d, published in the Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 26, at pages 2953-2971 (1988). Properties of such polymeric materials, including their low dielectric constants, are further discussed by R. Olson in xe2x80x9cXylylene Polymersxe2x80x9d, published in the Encyclopedia of Polymer Science and Engineering, Volume 17, Second Edition, at pages 990-1024 (1989).
Several parylene films have been developed for deposition within integrated circuits. Parylene-N is deposited from unsubstituted p-xylylene at substrate temperatures below about 70-90xc2x0 C. The parylene-N films typically do not adhere well to silicon oxide and other semiconductor surfaces. Furthermore, parylene-N films typically have poor thermal stability at temperatures above about 400xc2x0 C. Thermal stability of parylene films is improved by fluorinating or chlorinating the cyclic dimer of p-xylylene to make parylene-F films or parylene-C films. However, the substituted p-xylylene cyclic dimers are even more expensive than the unsubstituted cyclic dimer and are more difficult to process. Copolymers of p-xylylene and fluorinated or chlorinated monomers may also improve thermal stability. However, the fluorine or chlorine within the films can corrode metal electrical interconnects when an electrical bias is applied.
The p-xylylene cyclic dimer typically used as the parylene precursor is formed as a solid that is separated from the reactants and other products during formation of the cyclic dimer. Although the cyclic dimer is very expensive, the cyclic dimer provides a controllable deposition process for making parylene films having low dielectric constants. The cyclic dimer is produced from p-xylene, or a derivative thereof However, the cyclic dimer is a solid material at room temperature, and is difficult to handle as a chemical precursor in an integrated circuit manufacturing environment.
There is not a suitable method reported in the literature for depositing parylene from p-xylene, which is a liquid at room temperature and can be conveniently handled as a chemical precursor in an integrated circuit manufacturing environment. A process that deposits parylene from p-xylene, or a derivative thereof, without formation of an intermediate cyclic dimer would avoid a considerable number of intermediate processing steps. Such a process would preferably prepare a high yield of p-xylylene.
U.S. Pat. No. 4,438,021, issued Mar. 20, 1984, describes a catalyst composition for dehydrocoupling of toluene or xylene to form a high yield of linear dimers. Such linear dimers are inferior to p-xylylene for formation of polymers films on substrates. Any alternative process for making p-xylylene would preferably minimize formation of linear dimers. There remains a need for a process and apparatus compatible with integrated circuit manufacture that converts p-xylene, and derivatives thereof, to p-xylylene.
The present invention provides a method and apparatus for depositing a low k dielectric layer from a source of p-xylene with minimum formation of side reaction products. In particular, a method and apparatus is provided for in-situ production of p-xylylene, or derivatives thereof, from p-xylene, or derivatives thereof, at conditions that substantially favor formation of p-xylylene. A suitable apparatus and method includes a platinum or palladium coiled surface sized for catalytic dehydrogenation of p-xylene at a temperature between about 400 and about 900xc2x0 C. with a residence time of about 1 to 100 milliseconds, a flow rate of between about 5 and about 200 sccm, and a deposition chamber pressure between about 30 and about 500 millitorr. Another suitable apparatus and method provides for plasma-energized dehydrogenation of p-xylene using from about 10 to about 1000 W of constant high frequency RF power, or from about 20 to about 2000 W of pulsed high frequency RF power. After formation of the p-xylylene, a homopolymer or a copolymer can be deposited on a substrate in a vapor deposition chamber with or without addition of RF power. Alternatively, p-xylene can be dehydrogenated in the deposition chamber by application of the RF power above the surface of the wafer, using from about 10 W to about 80 W of constant high frequency RF power, or from about 20 W to about 160 W at pulsed high frequency RF power, which may then react at the substrate surface with an optional comonomer. The present invention also provides a method and apparatus for in-situ production of p-xylylene, or derivatives thereof, from 1,4-bis(formatomethyl)benzene or 1,4-bis(N-methyl-aminomethyl)benzene.